Experiments on voltage-clamped internally perfused squid axons indicate that internal alkaline earth cations can suppress the inward sodium current in low internal strength solutions. Internal barium ion and, to a lesser degree, external barium ion reduces the current in the delayed rectifier potassium conductance. Both inward and outward currents through the potassium channel are blocked. Experiments on internally dialyzed barnacle muscle fibers indicate that the divalent anion SITS blocks chloride efflux irreversibly when applied to the outside. Internal SITS also blocks chloride efflux irreversibly but also causes a 10-20 mV depolarization. Neither procedure blocks chloride influx. Experiments on voltage and current clamp giant neurons indicate that calcium injection over a wide range of concentrations induces a potassium conductance increase that varies as the log of the calcium concentration. The conductance increase is prolonged in the cold and is not suppressed by lithium replacement of sodium. The light response appears to be mediated by an internal release of calcium with subsequent activation of a potassium conductance. The potassium current is voltage dependent. It is reduced by tetraethylammonium and is unaffected by rubidium in the external solution.